The present invention relates to a surveying instrument.
In a case where a surveying instrument is installed for performing a surveying operation, a true north is measured for setting a reference direction of the surveying instrument. As one of the methods to determine azimuth at a position of installation, there is a method that the true north is surveyed by utilizing the sun.
A position (i.e. latitude and longitude) where the surveying instrument is installed is already known, the sun is sighted by a telescope from a position or installation and a horizontal angle of the sun when the sighting is performed is obtained. Further, the true north can be surveyed by obtaining an azimuth angle of the sun based on the latitude, longitude and time moment when the sighting is performed and replacing (updating) the horizontal angle by the azimuth angle.
As a means to measure the true north, after the installation of the surveying instrument, the sun is sighted by a telescope as provided on the surveying instrument, and then, a vertical angle and a horizontal angle in the sighting direction are measured. For this reason, a time difference occurs between a moment when the sun is sighted and a moment when a directional angle is measured. The sun moves at a high speed (15 arcseconds/second at maximum) due to the rotation of the earth, and the horizontal angle as measured is deviated, at such an extent as obtained by the time difference multiplied by the speed of the sun, from the actual position of the sun.
For this reason, in a case where it is necessary to measure the true north with high accuracy, since the deviation of the position of the sun appear as an error, it has been desired that a true north surveying which does not include the deviation of the position of the sun in a measurement result can be performed with higher accuracy.